Andy,
Regarding your questions-
The 'cracks' in the stock photos are present but very minor. There are no apparent indications of repair to the stock, and I'm able to view it in hand, not via photos. There's a very small and clean repair to the side of the hand guard, but it requires a close look. I'm confident we can rule out the idea that this is a Hungarian 31M, it lacks the expected markings on both the stock and the metal (receiver and barrel). I still think Yugoslav repair (possibly) and reuse (likely) are the most likely scenarios.
For your information, there are very faded numbers where you observed, in typical M.95 style (numbers followed by a single letter), exactly what I'd expect from a carbine that went through some level of refurbishment. There is a Steyr 'K' stamp on the butt plate, but no unit designation. The inside of the plate is stamped.